Abrasive tool

Abstract: The invention provides an abrasive tool comprising a bond material having dispersed therein abrasive grains comprising at least one superabrasive component and microcrystalline alumina filamentary particles with an essentially uniform orientation as well as segmental wheels comprising same.

Description: This invention relates to cutting wheels. Tools containing superabrasives are widely used for cutting extremely tough materials such as concrete for example. It is found however that the cost of such wheels is very high because the superabrasive component itself is very expensive. There is therefore considerable interest in the production of tools that are very effective and at the same time less expensive than tools in which the superabrasive component provides 100% of the abrasive content. One such approach is illustrated in US Patents 5,152,810 and 4,944,773 in which part of the superabrasive component is replaced by a sol-gel alumina abrasive with surprisingly advantageous results and a significantly lowered cost. EP-A-0 395 087 discloses cut-off wheels in which the abrasive filament is encased in a bond system. Example V of said document describes a resin-bonded cut-off wheel. Such a wheel is a thin disc of uniform thickness consisting entirely of bonded abrasive with a relatively small mounting hub at the center. It is mentioned in said document that the orientation of the filaments in the abrasive article is not critical and in general there will be no dominant orientation unless special measures are taken. Furthermore, it is believed, in said document, that greatest efficiency will be achieved by orienting the filaments radially so that one end is presented at the cutting surface. The known products do not fulfil every need and they should therefore be further improved. It is therefore the object of the present invention to provide a cutting wheel having better characteristics than known products. This object is solved by the cutting wheel according to independent claim 1. Further advantageous features, aspects and details of the invention are evident from the dependent claims, the description and the drawings. The claims are to be understood as a first non-limiting approach to define the invention in general terms. The present invention provides a further advance over known products by providing the sol-gel alumina in a form conferring advantages in a highly efficient manner and adding new possibilities not described in the prior art. The present invention provides a cutting wheel comprising an abrasive tool comprising a bond material having dispersed therein abrasive grains comprising at least one superabrasive component and filamentary particles of a microcrystalline alumina having an essentially uniform orientation. The filamentary particles are essentially uniformly oriented by which is meant that their longitudinal axes are aligned such that the majority, and more preferably at least 75%, lie within a 120° arc and more preferably within an arc of about 60°. The filamentary particles have a substantially uniform cross-sectional shape along an axis of symmetry and have preferably an aspect ratio (defined as the ratio between the length along the axis of symmetry and the greatest dimension perpendicular to that axis) of at least about 3:1 and more preferably at least about 10:1. In some cases the filamentary particles can be much longer and even joined to form a continuous filament for at least the pre-use configuration of the tool. In the event a continuous filament is used to make the abrasive tool, the filament is laid in multiple tight folds with the sides of the folds essentially parallel in the desired direction of orientation of the filamentary particles. Such a continuous filament behaves essentially as a plurality of individual filaments, and for this reason is understood to fall within the essential scope of this invention. The cross-sectional shape of the particles can be anything convenient but the most easily fabricated shapes are round or roughly square. Nevertheless the utility of the invention is not constrained by the shape of the cross-section. The particles comprise a microcrystalline alpha alumina by which is meant that the individual crystallographically distinct domains or crystallites that make up the particles have an average diameter (as measured by the average length of an intercept line drawn across a cross-section of the particle) of less than about 10 µm and preferably less than about one micrometer. The particle can contain other components such as phases comprising magnesia, zirconia, spinels, and rare earth metal oxides but is comprised of at least about 60% by weight, and more preferably at least 90% by weight, of alpha alumina. The density of the particles should be at least about 90%, and preferably at least 95% of the theoretical density for the composition. The most preferred particles have a hardness of at least 18 and more preferably at least 20 GPa. It is often desirable that the filamentary particles contain other components modifying their abrasive properties. For example finely divided abrasive particles, such as diamond, CBN, tungsten carbide and the like, can be incorporated. Other particulate matter that can be incorporated might include high temperature-stable lubricants such as boron nitride (hexagonal form), molybdenum sulfide and graphite, grinding aids such as metallic tin and other fillers. Such additions are preferably chosen to ensure that their quantity and physical properties do not excessively reduce the hardness and strength that characterize the unmodified solgel alumina filamentary particles. Such particles are made by a sol-gel process in which a sol of an alpha alumina precursor is gelled, usually by addition of an acid, dried, then fired. The size of the crystallites in the particle can be reduced by addition of a material effective to nucleate the crystallization of the alpha alumina from the precursor phase. Such materials are usually isostructural with the alpha alumina phase that is being formed with lattice parameters that are as close as possible to those of alpha alumina itself. Alternatively the crystallite size can be limited by the use of pinning agents that restrain the growth process during the firing to form alpha alumina. Included among patents teaching suitable methods of making the filamentary particles are US-A-4,314,827; US-A-4,632,364; US-A-4,744,802; US-A-4,770,671; US-A-4,881,951; US-A-4,954,462; US-A-4,964,883; US-A-5,053,369; US-A-5,076,815; US-A-5,114,891; US-A-5,139,978; EP-A-408,771 and PCT Application 92/01646. The particles can be made in any convenient manner but the most accessible route is through extrusion. In such a process the alpha precursor is extruded as a gel and then dried and fired to form the filaments. One suitable apparatus for producing such filaments is described in US-A-5,090,968. It is also possible to extrude and dry the filaments to a point at which they can conveniently be handled, incorporate them in the tool as it is formed and fire them in situ in the abrasive tool. The orientation of the filamentary particles in the tool is particularly important in determining the types of properties that the tool will have. Especially in wheels designed for cutting, as, for example, a concrete cutting segmental wheel, the axis of symmetry of the filaments being essentially parallel to the cutting surface but located at the edges thereof is particularly useful. Conventional concrete cutting wheels tend to wear at the edges such that the width of the cut made will decrease as the wheel wears. Filamentary particles lying parallel to the cutting surface will provide even wear properties, inhibit segment erosion at the edges, and keep the cutting surface square. In such situations the filamentary particles are acting to reduce wear rather than as abrasives in themselves. The shape is that of a wheel with the abrasive located at least at the periphery. Because of the cost of the superabrasive and because the wheels are conventionally used in situations where the wheel is under great stress, it is common to use a solid metal plate as the core portion of the wheel with the abrasive located at the rim, often in the form of segments attached to the core portion. The bond in which the abrasive is held in such cases is a metal. The means of attachment is not critical providing it affords a strong, permanent attachment to the core portion. Suitable means include welding, brazing and sintering. The invention is now described in more detail with reference to the attached Drawings which are for the purpose of illustration only. Figure 1 is a cross-section of a segment designed for attachment to a segment wheel, wherein the filamentary abrasive 7 is laid in tight fanfolds parallel to the grinding surface. Figure 2 is a top view of the segment of Figure 1 after a period of wear to expose the filamentary abrasive particle 7 laid parallel to the sides of the segment and adjacent the edges. Figure 3 is a cross-section of the segment shown in Figures 1 and 2 taken perpendicular to the cross-section of Figure 1. The section is of a used wheel showing in exaggerated form that the filamentary particles 7 cause the segment to wear to a U-shape and thus maintain its width of cut rather than becoming rounded in profile. Other configurations with potential advantages can be devised combining the abrasive power of superabrasives and the particular structural advantages that flow from the use of the filamentary particles. These include for example forming the filamentary abrasive into an interlaced ring rather in the manner a ball of string may be wound with preferably a more flattened profile and then forcing a shapeable mixture of bond plus superabrasive to fill the interstices in the interlaced ring and then curing, firing or otherwise causing the bond material to become rigid in the form of an abrasive tool such as a wheel. Such a structure could be given particular advantages by causing the dimensions of the interstices to be of such a size as to entrap grains of superabrasive and thus present them in a very rigid retaining structure to the surface to be ground or cut. The cutting wheel of the invention has incorporated into the structure filamentary particles formed from a microcrystalline alumina which confer significant advantages.

Claims: A cutting wheel comprising an abrasive tool located at the rim of said cutting wheel, said tool comprising a bond material being a metal bond and having dispersed therein abrasive grains comprising at least one superabrasive component selected from the group of diamond, cubic boron nitride (CBN) or tungsten carbide and microcrystalline alumina filamentary particles with a uniform orientation wherein said filamentary particles are oriented parallel to the cutting surface and adjacent lateral edges thereof. The cutting wheel according to claim 1 in which the filamentary particles incorporate finely divided particles of an additive selected from abrasives, high temperature-stable lubricants and grinding aids. The cutting wheel according to claim 1 or 2 in which the abrasive tool is in the form of segments.

Inventors: FRODIN JAMES EUGENE, KRAMP JEAN, PELLOW SCOTT, FRODIN, JAMES EUGENE, KRAMP, JEAN, PELLOW, SCOTT
Applicants: NORTON CO, NORTON COMPANY
Application year: 1994
Main classification: B24D3
IPC codes: B24D5, B24D3, C04B35, B28D1